The Lives of Stars

1
The Lives of Stars

• Chapter 12

2
Life on Main-Sequence

• Zero-Age Main Sequence (ZAMS)
• main sequence location where stars are born
• Bottom/left edge of main sequence
• H fusion begins
• As star ages
• Energy source is H fusion
• composition changes
• H -gt He
• Location in H-R diagram slowly changes
• begins to move away (right/up) from ZAMS
• broadens (smears out) main sequence

3
Stellar Lifetimes

• 90 of stars life spent in main sequence
• Lifetime depends on mass

4
Main Sequence to Red Giant

• H in core used up
• He ash in core
• no more fuel for energy
• Gravity begins to win
• core contracts, gets hotter
• start H fusion in shell surrounding core
• outer layers expand
• Star becomes Red Giant

5
Further Evolution

• As core contracts
• temperature increases
• becomes hot enough
• Begins to fuse He into C
• Energy production
• stops core collapse
• star is stable again

6
Beginning of the End

• When He exhausted, star out of fuel again
• core collapse resumes
• He shell burning begins
• outer layers expand
• Star becomes Red Supergiant
• strong mass loss occurs via stellar wind

7
Stellar Evolution
8
Evolution of Massive Stars

• Up to C-O core, evolution same for all stars
• From then on, different paths
• Low-mass stars
• no C burning
• core energy generation complete
• star dies
• High-mass stars
• C burning begins in core
• Eventually fuse heavier and heavier elements

9
Making Heavy Elements

• High-mass stars fuse heavier elements in
  cores C -gt Ne -gt O -gt Si -gt Fe
• at each step, core collapses further
• This nucleosynthesis produces most elements up to
  iron (Fe)

10
Evolutionary Tracks
11
Star Clusters

• Star clusters
• Stars born same location, same time
• contains stars with different masses
• permits study of stellar evolution
• Age of cluster
• determined by which stars have departed main
  sequence

12
Globular Clusters

• spherical ball of stars
• concentrated toward center
• 10,000 - 100,000 stars
• about 150 around our Galaxy
• very distant from Sun (gt10,000 LY)
• sizes 50-300 LY diameter

13
Open Clusters

• 100s of stars (up to 1000)
• smaller than Globular clusters
• no central concentration
• Found within the Galaxy
• 1000s known
• diameters lt 30 LY

14
H-R Diagrams of Clusters

• Cluster ages are different
• globular clusters oldest
• open clusters relatively young
• H-R diagrams indicate age
• interpret using stellar evolution theory

15
Cluster Evolution
16
Estimating Cluster Ages

• Make H-R diagram for cluster
• Have all stars arrived at ZAMS?
• if not, cluster extremely young
• Have some stars departed Main Sequence?
• cluster is older
• main sequence turn-off point
• determines cluster age
• the farther down the turn-off, the older the
  cluster

17
Theoretical
NGC 2264
Theoretical
47 Tuc
18
Ages of Clusters

• Globular clusters
• only lowest part of main sequence is present
• typical age 15 billion yrs
• Open clusters
• much younger than globulars
• all ages 1 million yrs up to a few billion yrs

19
Stellar Death
20
Death of Stars

• Two possibilities
• Low mass stars lt 5 Msun
• end is planetary nebula ? WHITE DWARF
• High mass stars
• end is type II supernova ? either NEUTRON
  STAR or BLACK HOLE

21
Fate of Low Mass Stars

• During end of red supergiant phase
• large mass loss
• star loses entire envelope, revealing core
• core becomes white dwarf
• white dwarf slowly cools
• eventually becomes black dwarf

22
Planetary Nebulae

• During transition to white dwarf
• outer layers expanding
• exposes hot core
• shell material heated begins to glow
• Result is a planetary nebula
• tens of thousands known in our Galaxy

23
Planetary Nebulae
24
Evolution to White Dwarf

• Low mass stars
• cannot fuse carbon
• lose energy source
• gravity wins
• core contracts
• Core contraction
• produces very high density
• electron degeneracy pressure
• stops core collapse
• remnant core becomes white dwarf

25
White Dwarf Stars

• Properties
• diameter same as Earth
• very dense (1 tsp several tons!)
• very hot on surface
• Chandrasekhar limit
• Maximum mass 1.4 Msun
• larger stars collapse

Diamond stars??
26
Fate of Massive Stars

• High-mass stars fuse heavier elements in
  cores C -gt Ne -gt O -gt Si -gt Fe
• at each step, core shrinks further
• fusion stops when iron (Fe) produced
• This nucleosynthesis produces most elements up to
  iron

27
Collapse and Explosion

• When core mass exceeds 1.4 Msun
• collapse continues unabated
• all protons converted into neutrons
• collapse abruptly halted by neutron degeneracy
  pressure
• results in shock wave explosion
• Produces type II supernova
• Some material falls onto core
• M lt 2.5 Msun neutron star remains
• M gt 2.5 Msun black hole produced

28
Supernovae

• Supernovae as bright as entire galaxy
• Ejection velocities
• millions of miles/hr (10,000 km/s)
• Supernova explosion
• heavy elements (C, N, O, Fe) returned to
  interstellar medium for recycling
• also produces elements heavier than iron
• elements such as gold, silver, uranium

29
Pulsars

• Pulsating radio sources
• Periods .001-10 seconds
• Very regular
• also observed in optical (crab nebula)
• Pulsars spinning neutron stars
• fast period requires very small objects
• neutron stars only possibility
• Radiation and particles beamed out from magnetic
  poles
• spinning lighthouse effect results in observed
  pulses

30
Novae

• Novae NOT same as Supernova
• less energetic not as bright
• Binary system with mass transfer onto WD
• material accumulates on WD surface
• eventually nuclear detonation occurs
• result is a nova

31
White Dwarf Supernovae

• As mass accumulates, WD exceeds Chandrasekhar
  limit
• rapid core collapse occurs
• Resulting explosion Type I supernova
• Properties somewhat different than Type II SN
  (caused by massive star explosions)